Active Deformation Front Research Articles

Detachment folding, growth mechanism and seismic potential in the Jammu Sub-Himalaya

The Surin Mastgarh anticline (SMA) marks the active deformation front in the northwest Sub-Himalaya near Jammu. The southern limb of the SMA is not truncated by an emergent thrust, unlike other frontal folds along the Himalayan front. We investigate the structural data, deformation pattern, and seismic potential of the SMA using field surveys and re-interpretation of seismic profile. Strath profiles along the Chenab and the Munavar Tawi river valleys indicate active growth of the SMA by layer parallel shortening, and limb rotation accompanied by flexural slip, suggesting an early phase of its development. Lateral migration of the southern limb suggesting limb lengthening and bending at the hinge are the dominant mechanisms of fold amplification with strain localization at the fold core above the floor thrust tip. Bending moment faulting at the hinge led to the formation of crestal grabens or lakes in the southeastern section of the SMA. Our results suggest the existence of a weak, less viscous layer beneath a brittle sedimentary detachment. The SMA initiated as a detachment fold and sequentially deformed by passive roof thrusting. Therefore, the seismotectonic model of the Jammu Sub-Himalaya is different from that of the central and eastern Himalaya.

Geomorphic expressions of active tectonics across the Indo-Burma Range

The Indo-Burma Range (IBR) is an accretionary complex associated with the oblique subduction zone between India and the Burma microplate. It remains an outstanding question whether the ongoing plate-boundary deformation involves active subduction of the Indian plate. To study the tectonics in this region, we investigate drainage divide stability and the topographic expression of active tectonics across the IBR. We extract elevation data from the 30 × 30 m SRTM digital elevation model and identify streams, drainage divides and basins. We select the divide segments to construct and evaluate divide stability using the Gilbert metrics and χ values of the channel heads located on both sides of the divide portions. We calculate several geomorphic indices, including basin-scale (hypsometry and relief) and stream-scale (normalized steepness index [ksn]) measures to reveal tectonic uplift patterns. We find that most of the divide segments are static, allowing us to study the morphometry of streams and drainage as indicators of spatial variation in tectonic uplift in the context of large-scale variations of precipitation and lithology. Our morphometric results support ongoing tectonic uplift distributed across the inner belt of the IBR, and they cannot be explained by variations in precipitation or lithology. The eastern region is characterized by higher relief, hypsometry and ksn compared to the western outer belt. While the active deformation front lies far to the west, the inferred higher uplift rates in the eastern range suggest active out-of-sequence thrusts in the inner belt of the IBR that splay off from the active megathrust.

Morphotectonic Evolution of the Siwalik Hills between the Yamuna and the Markanda River Exits, NW Himalaya

ABSTRACT The Main Frontal Thrust (MFT) marks the present day active deformation front of the Himalaya and separates it from the Indo- Gangetic plains. The dynamic growth of the outermost Siwalik hills due to thrusting along the MFT has a direct influence on the drainages which collectively control the landform evolution of the region. In this study, the Siwalik hills (called as the Dhanaura range in the study area) between the Markanda and the Yamuna river exits in the NW Himalaya is investigated for its morphotectonic evolution using geomorphic indices, longitudinal river profiles, and topographic profiles. The results suggest presence of at least four structures that have merged to form the Dhanaura range. The first expression of the Siwalik hills was marked by the uplift of the surface due to a blind thrust in the northwestern part of the study area, followed by the growth of the Pataliyon anticline along a northeastern segment. The growth of the Dhanaura anticline occurred later by the merging of three MFT segments in the southern part of the study area. The Dhanaura anticline formed a barrier to the drainages arising from the initial northern topography, forcing them to get deflected. There also exists an unidentified structure to the north of the Dhanaura anticline as evident from the longitudinal river profiles, structural data, and drainage network of the area. Low mountain front sinuosity ratio, moderate hypsometric integral (HI) values and tilting of drainage basins in the study area suggest the structures are active.

Major temporal variations in shortening rate absorbed along a large active fold of the southeastern Tianshan piedmont (China)

The investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range. Here, we estimate the shortening rate absorbed by a Quaternary emergent detachment fold on the southeastern piedmont of the Tianshan (China). Our work is primarily based on new 10Be cosmogenic exposure dating of deformed alluvial surfaces. The method we have developed combines depth profiling with sampling of surface cobbles, thereby allowing exposure time, erosion rate and inheritance to be simultaneously constrained. The exposure ages of the uppermost uplifted alluvial surfaces are around 140±17 ka, 130±9 ka and 47±9 ka, from west to east. A terrace lying below the 140 ka surface is dated at 65±5 ka. The ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods: (1) the excess area method, where sedimentation rates, extracted from magnetostratigraphic studies, are used to determine the amount of sedimentation after the abandonment of the river; and (2) a folding model derived from sandbox experiments. The late Pleistocene shortening rates are shown to be between 0.4±0.1 mm/yr and 0.8±0.5 mm/yr on the western part of the fold and 2.1±0.4 mm/yr along its central part. The central part of the frontal Yakeng anticline therefore accommodates up to 25% of the total shortening currently absorbed across the whole Eastern Tianshan range (8 mm/yr). However, this situation seems to have prevailed for only the last 150 ka, as the shortening rate absorbed by this nascent fold was previously ten times slower. While the initiation of folding of the Yakeng anticline can be traced back to 5.5 Ma ago, the basinward migration of the active deformation front onto the Yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east, probably in relation to the tectonic activity of the folds in the hinterland.

Shear-wave velocity model of the Chukuo fault zone, Southwest Taiwan, from cross correlation of seismic ambient noise

The Chia-Nan (Chiayi-Tainan) area is in the southwestern Taiwan, and is located at the active deformation front of the collision of the Eurasian continental plate and the Philippine Sea plate, which causes complex folds as well as thrust fault systems in the area. The Chukuo fault zone is a boundary between the Western Foothill and the Western Coastal Plain in the Chia-Nan area. The nature of the crustal structure beneath the fault zone, especially the eastern part of the fault zone with mountain topography, has not been well known in detailed due to lack of drilling data as well as its limitation in using other geophysical methods, such as active source survey. In this study, we deployed an array with 11 broadband seismic stations to monitor the seismicity of the Chukuo fault zone. The array has recorded more than 1000 microearthquakes around this area. It provides an opportunity to use P- and S-wave travel time data to investigate the both the crustal P- and S-velocity in the fault zone, however due to the nature of the earthquake distribution, the ray density is relatively low at depth between 0 and 7km. In addition, the uncertainty of S-wave reading for small earthquake also a limit in building precise S-velocity profile, Thus, we take the advantages of using cross-correlation of seismic ambient noise to investigate crustal S-velocity profile in the Chukuo fault area, especially in the mountain area where crustal faulting is a dominated phenomenon. The results indicate that S-wave velocity in the uppermost crust in the Chukuo fault zone is shown to be slower than previous studies. A low velocity layer exists at depth between 1 and 2km in the east of the Chukuo Fault. The low S-velocity is related to a highly fractured upper crust due to intensive deformation caused by the orogenic process.

Active deformation front delineated by drainage pattern analysis and vertical movement rates, southwestern Coastal Plain of Taiwan

Orogenesis in Taiwan results from an arc-continent collision between the Eurasian plate and the Philippine Sea plate. The southwestern part of Taiwan absorbs active shortening as evidenced by rapid crustal deformation and frequent earthquakes. Southwestern Taiwan may be subdivided into three tectonic–geomorphic domains, including the plain (PL), the tilted tableland (TT), and the low hills (LH), based on geomorphology, drainage pattern and stratigraphic records. The PL domain is the westernmost and is characterized by meandering rivers with relatively low surface slope and river gradient, but high river sinuosity. The TT domain has slightly tilted terraces with river meanders and many tributaries are developed on its surface. Anomalous river sinuosity occurs as a result of active tectonics. The LH domain has higher elevation and deformed geomorphic surfaces associated with a dissected landscape. This domain also has high stream gradients with low sinuosity which may indicate active deformation. We define crustal deformation, using Holocene deposits and radiocarbon dating in addition to geomorphic analysis that suggests a blind thrust may exist between the PL and TT domains. This blind thrust may indicate the position of the active deformation front of the orogenic belt of southwest Taiwan.

Structural evolution of the Latakia Ridge and Cyprus Basin at the front of the Cyprus Arc, Eastern Mediterranean Sea

The active deformation front between the African and Anatolian Plates in the eastern Mediterranean is delineated by the Cyprus Arc. Present plate kinematic vectors and earthquake data suggest that the movement along the eastern extent of the Cyprus Arc is largely sinistral strike–slip. It is, however, widely accepted that subduction took place beneath Cyprus until the early Miocene. Subduction has since thought to have become impeded by the collision of the Eratosthenes Seamount with the Island of Cyprus. The deformation front of the Cyprus Arc is dominated by the Latakia Ridge, a steep narrow high, which merges gradually with the lower slope of the broad Hecataeus Rise in the west and extends to the northern Levantine coast in the east where the expression of the ridge changes to a number of narrow, northeast-trending ridges and basins. The Latakia Ridge has previously been interpreted as either a thrust culmination or a positive flower structure. This study uses high-resolution marine seismic reflection profiles to delineate the stratigraphic and structural architecture of the deformation front. From seismic profiles, four stratigraphic units bounded by major basin-wide unconformities can be identified. These units are Eocene–lower Miocene, Miocene, uppermost Miocene (Messinian), and Pliocene–Quaternary. The timing of deformation determined from growth stratal architectures on the flanks of the major structures show that two main phases of deformation characterize the structural history of the deformation front. Each of these phases represents a different kinematic regime: a compressional regime dominated during the Miocene, whereas a strike–slip regime dominated in the Pliocene–Quaternary.

Basement control on oblique thrust sheet evolution: seismic imaging of the active deformation front of the Central Andes in Bolivia

In the area of the Bolivian Orocline, we examine the deformation pattern associated with the active development of a new thrust sheet. A dense grid of reprocessed 2-D seismic lines from hydrocarbon exploration industry is interpreted and a 3-D simplified structural and kinematic model is deduced. In the Boomerang Hills, onlapping Paleozoic and foredeep sediments are detached from the underlying S-dipping basement. They are thrust northeastwards by less than 2 km. Two zones can be differentiated along the Andean deformation front: (1) a W–E to NW–SE striking frontal segment of predominantly orthogonal shortening, comprising a thrust and anticline system; (2) a WSW–ENE striking lateral zone of oblique shortening within a complex system of thin-skinned strike–slip faults and minor folds. The deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the Paleozoic basin. The observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography, which provides a near oblique ramp for the onlapping wedge of sediments. The SW–NE thrusting direction is divided into orthogonal and tangential components. These are accommodated by convergent and strike–slip structures, respectively, which sole into a common detachment horizon. The structural evolution of the new thrust sheet in the Bolivian Orocline is primarily controlled by the paleorelief of the Brazilian Shield because: (1) the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and (2) small asperities in/close to the top of the basement promote fault localization. The coincidence of a relatively high basement position and a structural high of the Eastern Cordillera leads to the conclusion that the shape of the Brazilian Shield also controls the structural evolution of the pronounced eastern border of the Bolivian Orocline.

The structure of the Africa‐Anatolia plate boundary in the eastern Mediterranean

New marine deep seismic reflection data from south of Cyprus to the Syrian coast provide images of the upper crustal structure of the Cyprean Arc indicating that the deformation is partitioned along strike‐slip fault systems distributed over a wide zone, rather than forming a sharp plate boundary between African and Anatolian plates. Three major submarine strike‐slip fault systems, tens of kilometers in length, are mapped, which follow bathymetric features and merge together toward the east. These structures exhibit the three‐dimensional characteristics typical of strike‐slip deformation zones throughout the seven pre‐stack depth‐migrated sections, including several sets of positive flower structures forming bathymetric ridges, and the intervening contemporaneous subbasins. Beneath the Plio‐Quaternary sediments (500 m thick) that are blanketing the whole area and that reflect only the main surface traces of the fault systems, the subbasins are of varied dimensions and have rapid lateral changes in the thickness of the sedimentary fill. They include two major unconformities that have been correlated throughout the data marked by the M and K reflections. The M reflection is well imaged above varied thickness of Messinian evaporites, and the K reflection commonly appears at the base of syntectonic Tertiary age sediments. Within the eastern Cyprean Arc the K reflection corresponds to the basement‐cover contact, indicating that the strike‐slip tectonic scenario may have been active since the uppermost Cretaceous or lowermost Paleogene times to present. The active deformation front of the Alpine belt in the eastern Mediterranean corresponds to a strike‐slip fault system that forms a 110° arc and coincides at the southern slope of the Hecateaus Rise, continuing along the Latakia Ridge to the Syrian coast. The mapped structures fit within a general kinematic framework of left‐lateral transcurrent deformation that transfers slip from the subduction zone southwest of Cyprus into the Dead Sea transform system in the east. This change in the mode of deformation at the Africa‐Anatolia plate boundary occurs toward the junction between African, Anatolian, and Arabian plates.

A traverse of the ionian islands front with coincident normal incidence and wide-angle seismics

Marine vertical reflection profiling with a powerful airgun source, augmented with a few wide-angle seismometer stations on land, has been applied along a 180-km line across the presently active deformation front of the subduction of the Ionian Sea plate beneath the western Hellenides. East of the Ionian islands, there is limited evidence for reflectivity down to 23 km interpreted as the base of a rather thin continental-type crust. Under the western slope of the islands, a major normal-incidence reflector dips eastward, first gently to the west of the islands, then more steeply under them. This reflector may be extrapolated southwestwards beneath a bulge that is thought to represent the modern pre-Apulian front located over the subduction zone. The continuity, signature, and geometry of this reflector suggest that it may act as the lower limit of the western Hellenides where they override the Ionian Sea quasi-oceanic crust, rather than just an intracrustal interface of a pre-Apulian crust. The location of the previous deformation front, the Ionian thrust proposed in previous models, can be constrained by the new seismic data. The new data raise the possibility of a larger area of evaporite mobility than previously considered, insofar as active block motion apparently related to halokinesis is recognized west of the Zakynthos anticline. Diapirism, décollement, and westward-directed over-thrusting in the pre-Apulian crust may have been brought on by Late Pliocene-Quaternary reactivation of regional extension associated with the separation of the Peloponnesus from northern Greece as it was captured by the Anatolian-Aegean rotation and associated also with the fast clockwise rotation of the Ionian islands as they were sheared off the Apulian domain to the southwest by the initiation of the Kefallinia transform.

Polyphase tectonics at the southern tip of the Manila trench, Mindoro-Tablas Islands, Philippines

The southern termination of the Manila trench within the South China Sea continental margin in Mindoro is marked by a complex polyphase tectonic fabric in the arc-trench gap area. Onshore Southern Mindoro the active deformation front of the Manila trench is marked by parallel folds and thrusts, grading southward to N50° W-trending left-lateral strike-slip faults. This transpressive tectonic regime, active at least since the Late Pliocene, has overprinted the collision of an Early Miocene volcanic arc with the South China Sea continental margin (San Jose platform). The collision is postdated by deposition of the Late Miocene-Early Pliocene elastics of the East Mindoro basin. The tectonic and geological framework of this arc, which overlies a metamorphic basement and Eocene elastics, suggests that it was built on a drifted block of the South China Sea continental margin.